In one class of common rail fuel injection system, a plurality of fuel injectors are fluidly connected via separate branch passages to a common rail that contains fuel pressurized to injection levels. An electrical actuator attached to each of the fuel injectors controls the timing and duration of each injection event. In one alternative, these electrical actuators are operably coupled to a needle control valve that acts to apply or relieve fuel pressure on a closing hydraulic surface of a needle valve member. The needle valve member moves to open and close the nozzle outlets to permit fuel injection and end injection events, respectively. In this type of system, fuel at injection pressure levels is always present within the fuel injectors, and around their respective needle valve members. However, injection does not take place until pressure on the closing hydraulic surface of the needle is relieved. Depending upon the particular fuel injector, the needle control valve can be positioned on the high pressure side upstream from a needle control chamber or on the low pressure drain side leading away from the needle control chamber. The closing hydraulic surface of the needle valve member is exposed to fluid pressure in a needle control chamber. While many of these types of fuel injectors have performed well and provided additional control over injection timing and quantity, they sometimes actually tend to end injection events too abruptly, causing an increase in undesirable emissions, particularly smoke emissions. In other words, engineers have observed that these supposedly more sophisticated fuel injectors can sometimes, and at some conditions, produce more smoke emissions than their simpler counterparts that rely upon a fuel pressure drop and the action of a biasing spring to close the nozzle outlets to end an injection event. In addition, depending upon the location of the needle control valve, these fuel injectors can sometimes suffer from chronic leakage problems due at least in part to the fact that they are always pressurized, even between injection events.
In another common rail fuel injector strategy, an admission valve either opens a nozzle passage to a high pressure supply passage connected to the common fuel rail, during an injection event, or connects the nozzle passage to a low pressure drain passage between injection events. For instance, a Dutch Publication entitled, Common Rail Fuel Injection System For High Speed Large Diesel Engines, by Robert Bosch AG, © CIMAC Congress 1998 Copenhagen shows such a common rail fuel injector. It has a pilot operated three-way admission valve that fluidly connects the nozzle passage to either the high pressure supply passage or a low pressure drain passage. The nozzle passage is fluidly connected to the nozzle outlets when the needle valve member is lifted to its open position. The needle valve member in this injector appears to be a simple check valve, in that the needle valve member is biased towards a closed position with a pre-load on a biasing spring positioned in a vented chamber. Thus, the opening and closing of the nozzle outlets is controlled by fuel pressure in the nozzle passage that is acting against a simple biasing spring. Although the strategy presented by this fuel injector may have promise, it appears to suffer from several drawbacks, the least of which being the reliance upon a pilot operated admission valve. In other words, an electrical actuator is operably coupled to move with a pilot valve member. Depending upon the position of the pilot valve member, a control surface on a slave valve member is either exposed to low pressure or high pressure to move the same to a desired position. Because of the additional moving parts and close dynamic coupling between the pilot valve and the slave valve, there appears to be substantial likelihood of difficulty in mass producing fuel injectors of this type to reliably behave similar to one another, as would be necessary in order to gain the full potential benefits of a fuel injector design.
In addition, those skilled in the art will appreciate that rate shaping in common rail fuel injection systems is problematic.
The present invention is directed to one or more of the problems set forth above.